Transistor lengths have become so small that current continues to flow when they are turned off, draining batteries and affecting performance. When the gate-source voltage, Vgs, of a metal oxide semiconductor (MOS) transistor is less than its voltage threshold, Vt, it is in the sub-threshold region. This is characterized by a exponential change in drain current with Vgs. Sub-threshold leakage currents are difficult to control and reduce in conventional nano-scale planar complementary metal oxide semiconductor (CMOS) transistor technology. As technology scales, sub-threshold leakage currents can grow exponentially and become an increasingly large component of total power dissipation. This is of great concern to designers of handheld or portable devices where battery life is important, so minimizing power dissipation while achieving satisfactory performance is an increasingly important goal.
Two-dimensional short channel effects in a typical prior art planar transistor structure, shown in Figure. 1, result in a sub-threshold slope on the order of 120 mV/decade to 80 mV/decade. An ideal slope would be approximately 60 mV/decade, as shown in FIG. 2. The low power supply voltages used in nano-scale CMOS circuits that are now on the order of 2.5 V exacerbate the problem.
The planar transistor of FIG. 1 is comprised of a substrate 100 in which two source/drain regions 101, 102 are implanted. A control gate 103 is formed over the channel region 105 in which a channel forms during operation of the transistor.
Future supply voltages are projected to become even lower, in the range of 1.2 V, as designers try to improve battery life and performance of electronic devices. At such power levels, there will not be enough voltage range to turn on a transistor. A significant voltage overdrive above the threshold voltage is typically required to turn-on a prior art transistor and turn-off the transistor sub-threshold leakage. This can be several multiples of the 100 mV/decade threshold voltage slope illustrated in FIG. 2. For good Ion/Ioff ratios, the sub-threshold leakage current needs to be at least eight orders of magnitude or eight decades below the transistor current levels when the transistor is turned on. With a 1.2 V voltage range, there will not be enough voltage swing to allow both objectives: high on current and low sub-threshold leakage to be accomplished with conventional planar devices.
Gate body connected transistors as previously described in CMOS circuits provide a dynamic or changing threshold voltage, low when the transistor is on and a high threshold when it is off. Another alternative is using dual gated transistors. Yet another alternative is surrounding gate structures where the gate completely surrounds the transistor channel. This allows best control over the transistor channel but the structure has been difficult to realize in practice. Another technique has been to re-crystallize amorphous silicon that passes through a horizontal or vertical hole. None of these techniques, however, can have a sub-threshold slope less than the ideal characteristic of 60 mV/decade for a convention MOSFET.
For the reasons stated above, and for other reasons stated below that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a device structure that has reduced sub-threshold leakage.